The concept that the interface between two fluids may exhibit rheological properties different from those of the bulk phases has been known for some time. Since this early observation, the rheological properties of interfacial films have been the subject of extensive theoretical and experimental investigations. Experimental studies suggest that the rate of coalescence of bubbles in foams and emulsions, and therefore the stability of foams and emulsions, is dependent on the rheological properties of the fluid-fluid interface. Interfacial viscosities are believed to contribute to the suppression of interfacial turbulence by surfactants. They are also considered to play an important role in mass transfer across fluid-fluid interfaces and in solvent extraction processes. In addition, interfacial rheological phenomena have been found to affect oil displacement in a variety of oil recovery processes including water flooding, micellar/polymer flooding, alkaline flooding, and steam or CO.sub.2 processes employing foam for mobility control.
In order to quantitatively assess the role of interfacial viscosity in fluid-fluid interactions, one must be able to accurately measure this property. Much effort has been devoted to devising procedures for the measurement of interfacial rheological properties. Only the more recently developed procedures have proven reliable as previous methods were plagued with difficulties. Four currently accepted procedures for measuring surface shear properties at gas-liquid interfaces are the disk, the knife-edge, the thin biconical bob, and the deep channel interfacial viscometers. For the first time, recently, agreement between viscosity measurements was obtained supporting the validity of these procedures.
The disk, knife-edge and thin biconical bob instruments share similar designs. In all three cases, one measures the torque required to hold the bob stationary as the disk containing the fluids rotates with a constant angular velocity. Small deflections of the torsion wire from rest (where the angular velocity of the dish is zero) are determined by reflecting a low power laser beam off a small mirror mounted on the bob. The theoretical analyses of these instruments does not rigorously account for the viscous effects in the bulk phases, but instead assumes that this viscous traction does not affect the interface. In one theoretical estimation of the viscous interaction effects in torsional surface viscometers, it has been found that this assumption of perfect slippage of the surface film can cause serious error for low surface viscosities of less than 1 surface poise. Hence, these methods are valid only for relatively large apparent interfacial viscosities such that the effects of viscous forces in the adjacent bulk phases can be neglected.
The most widely used method for the measurement of surface shear viscosity at gas-liquid interfaces is the deep channel surface viscometer. In this design, fluid motion is generated in a circular canal with fixed walls and ceiling by rotating the floor of the canal located at a known depth below the surface. A circular design was used to eliminate the surface pressure gradients associated with earlier linear canal viscometers. The velocity distribution of particles floating at the gas-liquid interface is measured and analyzed to calculate the surface shear viscosity.
Newer techniques have recently appeared. In one design for a surface shear viscometer of high sensitivity, an interfacial film is driven by contact with a rotating ring inserted in a narrow gap in the wall of a cylindrical vessel. The practical limits of the instrument were somewhat narrow.
In another design a longitudinal wave apparatus was developed which allows the measurement of a combination of surface rheological properties by analysis of surface waves at liquid-gas interfaces. This instrument does not measure the surface shear viscosity or dilational viscosity, but a combination of these properties. For the determination of surface dilational viscosity, this instrument must be used in conjunction with a surface shear viscosity apparatus such as the deep channel surface viscometer. The application of this apparatus has been extended to include measurements at liquid-liquid interfaces.
In a new procedure for estimating the total interfacial viscosity (shear plus dilational) of oil/water/surfactant sytems, the method involves the measurement of droplet-droplet coalescence rates in an inclined, spinning drop instrument commonly used for interfacial tension measurements. This procedure is much easier than those previously described, but offers only order of magnitude determinations.
To date, few designs exist which allow measurement of interfacial shear viscosities at liquid-liquid interfaces. Investigators have extended the analysis and applicability of the deep channel geometry to liquid-liquid systems and have provided the theoretical analysis for use of the thin disk and biconical bob geometries at liquid-liquid interfaces. The deep channel procedure requires a complicated apparatus which is difficult to clean and tedious to use. Clean surfaces are essential for the measurement of surface properties, since minute quantities of contaminates can seriously affect results. Furthermore, the experimental difficulties are magnified for liquid-liquid systems when the top phase is opaque. The thin disk and biconical bob are more simple experimentally but, as stated earlier, are limited in sensitivity.
A recent procedure, the "cup-of-tea" method was developed for the measurement of liquid-gas surface shear viscosity. This method involves the observation of decaying surface motions of a cup of liquid following sudden cessation of rigid body rotation. Small particles are floated on the surface to serve as tracers. The rate of decay of these surface motions is strongly affected by the shear viscosity of the liquid-gas interface. The surface shear viscosity is determined from measurements of original cup angular speed, cup geometry, surface particle angular displacement, and bulk fluid properties. Angular displacement measurments for water/air and oil/air systems which have negligibly small surface shear viscosities show good agreement with those predicted theoretically.
The new, novel invention described in this patent application is a further development in the design of surface viscometers directed to the liquid/liquid case. This non-obvious, new device and corresponding, unique method of measurement has an experimental design much more simple, more easily cleaned and used, and less expensive to construct than prior designs. In addition, this novel, new method appears to be sufficiently sensitive and adaptive to have a wide range of applications.